This invention relates to a coating composition which uses a curable, fluoroolefin based copolymer as a basic component and is particularly suitable for application to polycarbonate resin surfaces and also for repairs of polyvinylidene fluoride based paint films.
Polycarbonate is excellent in various properties such as mechanical strength, dimensional stability, electrical insulation and heat resistance and has widely been used as an engineering plastic for electric and electronic parts, machine parts and housings, household utensils, and so on. Further, polycarbonate serves as a platic glass since this resin is very transparent and high in shock resistance, and recently the use of polycarbonate glass in buildings and automobiles is increasing.
However, polycarbonate is not sufficiently high in weather resistance and chemical resistance. When a polycarbonate article is used outdoors it deteriorates mainly by the action of ultraviolet rays in the sunlight to often result in the occurrence of yellowing, loss of transparency and/or crazing besides gradual lowering of tensile strength and elongation. As to chemical resistance, polycarbonate is susceptible to crazing and loss of transparency by the action of alkalis or organic solvents. Usually polycarbonate resins contain an ultraviolet absorbing agent for improvement in weatherability, but the effect is not always satisfactory, and the chemical resistance is not improved by the addition of an ultraviolet absorbing agent.
For some limited uses it is known to coat the surfaces of polycarbonate articles with another resin, such as acrylic resin, urethane resin or acryl-urethane resin, containing an ultraviolet absorbing agent thereby to improve both weatherability and chemical resistance, but this measure is not yet fully satisfactory since the resin used as the coating material is not excellent in weatherability.
JP-A 1-249839 shows a coating composition for application to polycarbonate surfaces. Essentially the coating composition is a solution of a fluroolefin based copolymer in an organic solvent and contains an ultraviolet absorbing agent and some other additives. Using this composition it is possible to form a coating film high in both weather resistance and chemical resistance on a polycarbonate surface. However, we have recognized that the adhesion of this coating film to the polycarbonate surface considerably weakens when the coated article is exposed to the sunlight for a long period of time. There is a possibility of overcoming this shortcoming by application of a suitable primer in advance of applying the fluoroolefin based paint, but this is an increase in the steps of a painting process and hence unfavorable for productivity.
Meanwhile, bake finish paints using polyvinylidene fluoride (PVD) as the principal component of the vehicle have been used as facing or top-coat paints. PVDF based paints give beautiful and long durable paint films and have largely been used for producing precoated metal sheets.
In factories, painting operations using PVDF based paints can be accomplished with good and stable quality of the products, but paints of this type are unsuitable for field painting because of needing baking finish. However, in practically using precoated metal sheets often there is the need of repairing the paint film because the paint film is liable to be locally damaged during handling and fitting of the precoated metal sheets. For repairing in fields it is impracticable to use a PVDF based paint for the aforementioned reason, and hence it is inevitable to use a paint of a different type. At present it is usual to use a paint of room temperature setting type, such as an acrylic resin paint or a fluororesin paint using a fluoroolefin based copolymer as the resin, for field repairs of PVDF based paint films. However, acrylic resin paints are generally insufficient in weatherability, and room temperature setting paints using a fluoroolefin based copolymer are not very strong in adhesion to PVDF based paint films.
A paint film formed by using a PVDF based paint is relatively low in surface energy because of a high content of fluorine. Therefore, in the case of coating this paint film with another paint for the purpose of repairing it is difficult to accomplish strong adhesion of the overlying paint film to the initially formed paint film. Sometimes the paint film to be repaired is ground precedent to the repairing painting to physically strengthen the adhesion of the repairing paint film, but this is troublesome and does not always give good results. From another aspect, the repairing paint is required to be comparative to PVDF based paints in the excellence of paint films in weather resistance, chemical resistance and soil resistance.